C Language for Interviews

Build a solid foundation by mastering how C stores, represents, and converts data — the starting point for every C program.

• Variable declaration syntax — type name; format, must declare before use in C

• Integer division — int/int truncates the decimal part, not rounded

• char data type — stores a single character, occupies 1 byte (8 bits)

• Floating-point types — float (single precision) vs double (double precision)

• Type promotion — automatic conversion rules in mixed-type expressions

• sizeof operator — returns size in bytes of a data type or variable

• Data type modifiers — short, long, signed, unsigned for range control

• const keyword — declaring non-modifiable variables at compile time

Master every operator in C — from arithmetic and relational to increment, ternary, and compound assignment — and understand their precedence.

• Relational operators — ==, !=, <, >, <=, >= return 0 (false) or 1 (true)

• Modulus operator — returns remainder of division (20 % 6 = 2), integers only

• Logical AND (&&) — short-circuit evaluation, both conditions must be true

• Pre-increment (++x) — increments first, then returns the new value

• Post-increment (x++) — returns current value, then increments

• Assignment operators — +=, -=, *=, /=, %= for compound assignment

• Operator precedence — *, /, % evaluated before +, -, parentheses override

• Ternary operator — condition ? value_if_true : value_if_false for inline decisions

Write accurate decision-making logic using the full range of conditional constructs C offers.

• if statement — executes a block only when the condition evaluates to true

• if-else if ladder — stops at the first true condition, remaining branches skipped

• switch statement — multi-way branch on integer or character values

• break keyword — prevents fall-through to the next case in a switch block

• default case — executes when no case matches, optional but recommended

• Nested if — if statements inside if blocks for layered conditions

• Logical operators in conditions — && (AND), || (OR), ! (NOT) for compound checks

Control repetition precisely using C's loop constructs and understand when to use each type.

• for loop — for(init; condition; increment), best when number of iterations is known

• while loop — condition checked before each iteration, may not execute if initially false

• do-while loop — executes at least once, condition checked after each iteration

• break statement — exits the loop immediately regardless of remaining iterations

• continue statement — skips the rest of the current iteration and moves to the next

• Nested loops — total iterations equal outer iterations multiplied by inner iterations

• Infinite loop — while(1) or for(;;) where condition is always true

Design modular, reusable C programs by mastering function declarations, parameter passing, recursion, and standard library usage.

• Function purpose — promotes code reusability, modularity, and readability

• Function prototype — declaration before use, typically placed before main()

• Call by value — a copy of the argument is passed, original variable is unchanged

• Recursion — function calling itself, used for factorial, Fibonacci, and tree traversal

• void functions — no return value, used for actions rather than calculations

• Standard library headers — stdio.h, math.h, string.h, stdlib.h and their functions

• Return type rules — always specify return type explicitly for clarity and correctness

Store and manipulate collections of data in C using fixed-size arrays, including 2D arrays and their relationship with strings.

• Array declaration — type name[size]; fixed-size collection of same-type elements

• Zero-based indexing — valid indices run from 0 to size-1

• Array initialization — int arr[] = {1,2,3}; or partial int arr[5] = {1,2};

• Out-of-bounds access — undefined behavior, no runtime checking in C

• 2D arrays — type name[rows][cols]; accessed with arr[row][col] notation

• Strings as character arrays — char str[] = "text"; null-terminated with '\0'

• sizeof on arrays — returns total bytes (number of elements × element size)

The most tested C topic in interviews — understand memory addresses, pointer arithmetic, and how pointers power arrays and functions.

• Pointer declaration — type* ptr; stores the memory address of another variable

• Dereferencing — *ptr accesses the value stored at the address the pointer holds

• NULL pointer — represents an invalid/null address, always check before dereferencing

• Pointer and array relationship — array name decays to a pointer to the first element

• Pointer arithmetic — ptr+1 advances by one element size, not one byte

• Double pointer — pointer to pointer (type** ptr), used for 2D arrays and out-parameters

• Dangling pointer — points to freed or out-of-scope memory, causes undefined behavior

• Function pointers — pointers to functions, used for callbacks and dispatch tables

Group related data using C's user-defined types and understand the critical memory difference between structures and unions.

• Structure declaration — struct name { members; }; groups related variables of different types

• Dot operator — used to access structure members via a struct variable

• Arrow operator (->) — used to access members through a pointer to a structure

• Structure vs union — structure stores all members simultaneously, union shares one memory location

• Structure size — sum of member sizes plus any padding added by the compiler

• Union size — equals the size of the largest member only

• Nested structures — structure within structure for hierarchical data representation

• Structure vs array — structures group different types, arrays group same type

Read from and write to files in C using the standard I/O library — essential for systems programming and embedded data logging.

• FILE* pointer — used for all file operations in C standard I/O

• fopen() — opens a file and returns FILE* or NULL on failure

• File modes — "r" (read), "w" (write), "a" (append), with "+" variants for read-write

• Error handling — always check if fopen() returns NULL before proceeding

• fprintf() — writes formatted output to a file, mirrors printf() for file streams

• fgetc() / fputc() — read or write a single character to or from a file

• EOF constant — signals end of file, used to detect when reading is complete

• fclose() — closes the file, flushes the buffer, and releases system resources

Manipulate text in C using null-terminated character arrays and the standard string library functions.

• String representation — array of characters terminated by '\0' (null character)

• strlen() — returns the length of a string excluding the null terminator

• strcpy() — copies a source string into a destination buffer

• strcat() — appends source string to the end of a destination string

• strcmp() — compares two strings lexicographically, returns 0 if equal

• strchr() — finds the first occurrence of a character within a string

• gets() danger — no bounds checking, causes buffer overflow (use fgets instead)

• Character vs string constant — 'a' is a char (1 byte), "a" is a string (2 bytes with '\0')